Lasers are widely applied in various applications in industry and their effective using is very important. Due to their physics features of creating the laser radiation the intensity profile of laser sources is described, typically, by the Gaussian function, FIG. 1. When focusing a Gaussian laser beam by a lens the intensity distribution in the focal plane of this lens is described by Gaussian function as well—this is well known feature of TEM00 laser beams. This intensity distribution is, also, characteristic one for the planes near the focal plane of a lens, thus, in the most interesting for real application working area around a focal point of a focusing lens the intensity distribution is characterized by the Gaussian function.
From one side, this Gaussian intensity distribution provides high energy concentration, especially when a laser beam is focused by a lens. However, from another side, for many scientific and industrial applications the Gaussian profile is not an optimum one because of non-uniform intensity distribution within a laser beam. In such laser applications like micromachining, engraving, scribing, drilling blind vias in PCB and many other applications, a uniform intensity profile of a beam is most preferable from the point of view of saving the energy and providing same conditions of material treatment by the laser radiation. For some laser applications, for example hole drilling, welding a donut or ring-shaped intensity distribution in the focal plane is a best choice.
Therefore, the task of re-distribution of energy within a focused laser beam to provide uniform, ring-shaped or other required intensity profiles is an actual industrial task; very often it is called beam shaping.
One of solutions used to transform the intensity distribution of focused laser beams is integrating beam shapers using arrays of microlenses, micromirrors, and or prisms to divide the source laser beam into small parts, beamlets; various implementations of this approach are described in Fred Dickey, et al., Laser Beam Shaping: Theory and Techniques, Marcel Dekker Inc., New York, 2000; and in WO/2005/085935 and U.S. Pat. No. 7,085,062. Light from all beamlets is then collected at a certain working plane by focusing with using additional optical components in such a way that each point of said working plane gets a portion of light from each beamlet. Thus, the final intensity of each point of working plane is defined by integration of light from all beamlets.
An obvious disadvantage of this integration approach is use of complicated, difficult to produce and expensive array optical elements. Another disadvantage of integration type systems is the strong speckle effect happening due to destroying the beam structure by splitting a beam, thus reducing its spatial coherence, and uncontrolled interference of light from multiple temporally coherent beamlets, this effect makes impossible to reliably create small focused beams of a size comparable with a wavelength of a beam, for example, of about 10 microns diameter in case of Nd:YAG or fiber lasers.
Another way of solving the problem is based on applying of diffractive optical elements (DOE) which have such a design that a beam passed through such an element provides uniform intensity in a certain location, examples of this method are presented in U.S. Pat. No. 5,864,430, U.S. Pat. No. 6,433,301, U.S. Pat. No. 6,791,060 and international patent application number WO/2007/034887. In case of applying the DOE the range of applications can be limited because of unacceptable diffraction losses and low resistance to high power laser beams.
One more approach is based on the well-known feature of a focusing lens to generate in its focal plane field amplitude proportional to Fourier-transform of the field amplitude function at the focusing lens input; this effect is sufficiently described by Joseph Goodman (Joseph W. Goodman Introduction to Fourier Optics, McGraw-Hill, New York, 1996) and in U.S. Pat. No. 6,975,458. Mathematical analysis on the base of diffraction theory shows that in order to get uniform intensity distribution of a spot in focal plane of a diffraction limited lens it is necessary to provide at the lens input the intensity distribution proportional to so called Airy disk, FIG. 2 described by function [J1(2πr)/(2πr)]2, where J1(2πr) is the first order Bessel-function of the first kind and r is the distance from the beam axis.
The approach illustrated by FIG. 3 and FIGS. 4a and 4b is described in U.S. Pat. No. 5,300,756, U.S. Pat. No. 6,639,177, U.S. Pat. No. 6,777,645, and U.S. Pat. No. 6,989,508 which implies creation of approximate to Airy disk intensity distribution from a laser beam by applying a binary phase plate 3 introducing a phase shift of half wavelength in a central region of an input Gaussian beam 1 and further focusing of that beam onto a target by means of a focusing lens 5. Because of this wavefront jump of half wavelength provided by the binary phase plate 3 the function of filed amplitude of the beam gets a jump as well, see FIG. 4a. The resulting field amplitude distribution approximates the Airy disk, but this approximation has evident disadvantages.
Due to the diffraction that the jumping wavefront shift leads to appearing in the final intensity distribution in the focal plane 6 of the focusing lens 5 not only a central spot but also sidelobes corresponding to 1st, 2nd and higher orders of diffraction as shown in FIG. 4b. Those sidelobes “contain” relatively high amount of energy and, in most laser technologies, are either useless or bring an unwanted effect on a workpiece. This is, evidently, loss of costly laser energy that can reach essential values especially in case of high power lasers.
Another disadvantage of this technical solution is in sharp edges of material of the binary phase plate 3 on the border of the regions where this wavefront shift occurs. In the case of powerful lasers, those edges become zones of overheating and lead to destruction of the binary phase plate, this is especially critical in case of high peak power short pulse lasers.
Conventional technical solutions are aimed at providing a uniform or flattop intensity profile of a laser beam; but very often performance of laser technologies can be improved by applying other profiles. For example, the laser technologies of drilling holes would benefit from using a ring-like profile; the donut intensity distribution is optimum in some welding applications, this is, for example, taught by Peter Haglund, et al. (Peter Haglund, et al. Surface tension stabilized laser welding (donut laser welding)—A new laser welding technique, J. Laser Appl., Vol. 25, No. 3, May 2013, 031501-1). Therefore, providing of various intensity profiles by the same beam shaping optics would bring advantages and flexibility to laser technologies.
Thus, from the point of view of modern requirements of beam shaping optics for real laser applications, the conventional solutions are not optimal.
What is needed to solve the above problems is a highly efficient method and an apparatus of beam shaping of focused laser beams featuring by suppressed losses of laser energy and capable to provide uniform, ring-shaped, donut or other intensity distributions in a focal plane of a focusing lens and in the area near this focal plane.